1. Field of the Invention
The present invention relates to a rolling bearing unit used for example for rotatably supporting a rotating shaft of a screw compressor.
2. Description of the Related Art
In order to support the rotating shaft of a screw compressor and the like, a rolling bearing unit such as shown in FIG. 2 has heretofore been used. This bearing unit is provided between an outer peripheral face of a rotating shaft 2 to which is secured a rotor 1 of a screw compressor, and an inner peripheral face of a housing 3, and is made up by assembling together a pair of first and second ball bearings 4, 5 of the angular type. With the present example, the so-called back-to-back or double back face assembly (DS) is adopted.
The directions of the contact angles .alpha..sub.1 the first and second ball bearings 4, 5 are made opposite to each other. Therefore, when the rotating shaft 2 is displaced towards the left in FIG. 2, the first ball bearing 4 on the right hand side in FIG. 2 supports the load in the axial direction (axial load), while when displaced to the right, the second ball bearing 5 on the left hand side supports the axial load, thus preventing displacement of the rotating shaft 2 and the rotor 1 relative to the housing 3.
With the rotating shaft 2 of the screw compressor in FIG. 2 however, an axial load F.sub.a is applied mainly in a direction as indicated by an arrow (left direction of FIG. 2) while a small axial load is only occasionally applied an the opposite direction. Consequently, if a pair of first and second ball bearings 4, 5 produced according to the same specification with respect to contact angle etc. are used with only the direction of the contact angles .alpha..sub.1 opposite to each other, then the life of the overall ball bearing unit is inadequate. The reason for this is as follows.
Due to the axial load F.sub.a applied in the one direction as mentioned above, the rigidity of the first ball bearing 4 on the side for Supporting the axial load F.sub.a (the right hand side in FIG. 2) is increased, while the rigidity of the second ball bearing 5 on the side which does not support the axial load F.sub.a (the left hand side in FIG. 2) is reduced. Accordingly, the result of applying the axial load F.sub.a to the first ball bearing 4, is to increase the Hertzian contact ellipse occurring at the contact regions between the rolling faces of the balls 6 of the first ball hearing 4 and the inner ring raceway on outer peripheral face of the inner ring 7 and between the rolling faces of the bells 6 of the first ball bearing 4 and the outer ring raceway on the inner peripheral face of the outer ring 8. The result of this larger contact ellipse being produced in the first ball bearing 4 on the side which carries the axial load F.sub.a (referred to hereunder sometimes as the axially loaded ball bearing 4), is to increase the rigidity of the first ball beating 4. FIG. 3 shows a general tendency of the relation between the size of an axial load F.sub.a acting on an angular type ball bearing, and the radial rigidity of the ball bearing. As is clear from FIG. 3, the radial rigidity of the first ball bearing 4 is expected to increase during operation of the screw compressor.
On the other hand, in the second ball bearing 5 which does not support the axial load F.sub.a, a contact ellipse is not produced in the case of no preload applied, or even if produced in the case of a preload applied, to is only small. As a result, the radial rigidity of the ball bearing 5 which does not support the axial load F.sub.a (referred to hereunder sometimes as the non axially loaded ball bearing 5) is less. Consequently, the majority of the radial load F.sub.r applied between the rotating shaft 2 and the housing 3 is supported by the first ball bearing or axially loaded ball bearing 4, while the second ball bearing 5 not only does not support the axial load F.sub.a but also provides only minimal support for the radial load F.sub.r. As a result, the load on the axially loaded ball bearing 4 is much larger than that on the non axially loaded ball bearing 5.
The life of the overall rolling bearing unit made up by assembling together the pair of first and second ball bearings 4, 5, is predominantly influenced by the shorter life one out of the pair of first and second ball bearings 4, 5. In the case where, as shown in FIG. 2, only the first ball bearing 4 is subjected to a large load while the load applied to the second ball bearing 5 is extremely small provided that the first and second ball bearings 4, 5 are made according to the same specification, then the life for the overall ball bearing unit is predominantly influenced by the life of the first ball bearing 4. Moreover, since the life of the first ball bearing 4 will not be sufficiently long, then the life for the overall rolling bearing unit becomes inadequate.
In view of this situation, there have heretofore been various attempts to extend the life of the rolling bearing unit fitted for example to a screw compressor. A first arrangement has been carried out wherein a positive gap or actual gap (in contrast to a negative gap under preload conditions) is provided rather than a preload being applied to the pair of ball bearings of the rolling bearing unit. When a positive or actual gap is provided in this way, then the contact pressure on the rolling faces of the balls and on the inner and outer raceways of the respective ball bearings is smaller than that for the case of a preload applied, so that the fatigue life of the rolling faces, as well as that of the inner and outer ring raceways is improved.
In the case of a rolling bearing unit assembled for example into a screw compressor and the like, which differs from a bearing unit where a preload is required to meet the demand for high rotational accuracy as with bearing units used for the shaft of a machine tool, a positive gap is applied between the respective ball bearings to thereby reduce the amount of heating during operation and thus improve the fatigue life.
With the construction example shown An FIG. 4, a portion of the housing 3 opposite to the outer ring 8 of the axially loaded ball bearing 4 is formed with a larger diameter so that a gap 9 exists between the outer peripheral face of the outer ring 8 and the inner peripheral face of the housing 3. Therefore, with this construction example, the radial load F.sub.r applied between the rotating shaft 2 and the housing 3 is supported only by the non axially loaded ball bearing 5.
With the construction example shown in FIG. 5 which is disclosed in Japanese Patent First Publication KOKAI No. S58160621, there is a change in addition to the construction shown in FIG. 4, Specifically, the contact angle .alpha..sub.2 of the non axially loaded ball bearing 5 is made smaller than the contact angle .alpha..sub.1 of the axially loaded ball bearing 4 (.alpha..sub.1 &gt;.alpha..sub.2). By making the contact angle .alpha..sub.2 of the non axially loaded ball bearing 5 smaller as with this construction example, the load capacity with respect to the radial load F.sub.r of the non axially loaded ball bearing 5 is increased.
With the abovementioned conventional constructions, however, a sufficient improvement in life is not always possible.
At first, in the case of the construction shown in FIG. 2 wherein the first and second ball bearings 4, 5 made according to the same specification are engagingly supported in the same manner between the outer peripheral face of the rotating shaft 2 and the inner peripheral face of the housing 3, then as mentioned before, the load on the first ball bearing or axially loaded ball bearing 4 is much greater than that on the second ball bearing or non axially loaded ball bearing 5, so that the life is shortened. Moreover in the case wherein a positive gap is provided inside the respective ball bearings 4 and 5, the life is improved compared to the case where a preload or a negative gap is provided. However from the viewpoint of the increase in load on the axially loaded ball bearing 4, this case is basically the same as for the preload case, and hence obtaining sufficient life improvement is difficult.
With the construction as shown in FIG. 4 wherein the radial load F.sub.r is not supported by the first ball bearing or axially loaded ball beating 4, since all of the radial load F.sub.r applied to the second ball bearing or non axially loaded ball bearing 5, then the fatigue life of the second ball bearing 5 tends to be inadequate. Furthermore, as a result of applying all the radial load F.sub.r to the second ball bearing 5, an internal axial load in the opposite direction to the axial load F.sub.a is produced in the second ball bearing 5. This internal axial load is applied together with the axial load F.sub.a to the first ball bearing 4, thus further increasing the axial load applied thereto, so that the fatigue life of the first ball bearing 4 also tends to be inadequate.
With the construction example as shown in FIG. 5 wherein the contact angle .alpha..sub.2 of the second ball bearing or non axially loaded ball bearing 5 is reduced, the load capacity with respect to the radial load of the second bell bearing 5 is increased so that the fatigue life of the second ball bearing 5 is improved to some degree. However considering that the radial load F.sub.r is supported by only the second ball bearing 5, then as with the construction example shown in FIG. 4, the fatigue life of the second ball bearing 5 tends to be inadequate. Moreover, although the internal axial load produced inside the second ball beating 5 is reduced with a reduction in the contact angle .alpha..sub.2, this internal axial load is still produced so that the fatigue life of the first ball bearing or axially loaded ball bearing 4 still tends to be inadequate.
Therefore, in order to ensure sufficient life of the rolling bearing unit, countermeasures such as increasing the size of the first and second bearings 4, 5, or using a high quality material for the bearings have heretofore been devised. However, with such countermeasures, the cost of the rolling bearing unit is increased, resulting in an increase in the cost of mechanical equipment such as screw compressors to which the rolling bearing unit is fitted.
Furthermore, in Japanese Patent First Publication KOKAI No. H5-280482 there is disclosed a rolling bearing unit where, in order to suppress the increase in internal axial load due to centrifugal forces, the contact angle of the first ball bearing which supports the axial load is made 30 to 40 degrees, while the contact angle of the second ball bearing which does not support the axial load is made 15 to 25 degrees.
However, with the construction disclosed in this publication, there has been no consideration with regards to the internal gaps and to the support of the radial load. Hence the fatigue life extension effect is not really adequate.